The present invention relates to a key for use in a cylinder lock with a rotatable key plug having a profiled key hole or keyway, said key comprising:                an elongated, substantially flat key blade having a longitudinal profile groove extending along at least a portion of the length of the key blade, with an inner wall of said groove being substantially parallel to a side surface of said flat key blade,        said longitudinal profile groove having an undercut portion adjacent to and inside a ridge portion of the key blade, the outside of said ridge portion forming a lower side surface of the key blade, at a lower part thereof,        said side surface at the lower part of said key blade lying substantially in the same plane as an upper side surface of the key blade at an upper part thereof, above said longitudinal profile groove, and        the inside of said ridge portion facing said inner wall of said groove.        
Such a key with an undercut groove is previously known, e.g. from U.S. Pat. No. 5,715,717 (Widén) or U.S. Pat. No. 5,640,865 (Widén). Such keys have proven to be very useful in that they provide an improved security. The key profile is quite distinguished from conventional keys, and it is rather difficult to copy such keys. Moreover, they permit a great variation of the cross-sectional profile, which is a great advantage.